In which of the following processes does the entropy decrease?

Options

A. Dissolution of NaCl in water.
B. The evaporation of water.
C. The conversion of `CO_2` gas into dry ice.
D. When a dozen of marbles are dropped on the ground.

Answer

To determine in which of the given processes the entropy decreases, we need to analyze each option based on the concept of entropy, which is a measure of the randomness or disorder of a system.

### Step-by-Step Solution:

1. **Dissolution of NaCl in Water:**
- When NaCl (sodium chloride) dissolves in water, it dissociates into Na⁺ and Cl⁻ ions.
- The ions are more randomly distributed in the solution compared to the ordered structure of solid NaCl.
- **Conclusion:** Entropy increases in this process.

2. **Evaporation of Water:**
- Evaporation involves the transition of water from a liquid state to a gaseous state.
- Gases have a higher degree of randomness compared to liquids due to the increased distance between molecules.
- **Conclusion:** Entropy increases in this process.

3. **Conversion of Carbon Dioxide Gas to Dry Ice:**
- In this process, carbon dioxide gas (CO₂) is converted into dry ice (solid CO₂).
- The gas molecules, which are far apart and highly disordered, become closely packed in a solid state, resulting in a more ordered arrangement.
- **Conclusion:** Entropy decreases in this process.

4. **Dropping a Dozen Marbles:**
- When marbles are dropped, they spread out and occupy a larger volume compared to when they are contained in a box.
- This spreading out increases the randomness of their arrangement.
- **Conclusion:** Entropy increases in this process.

### Final Answer:
The process in which entropy decreases is the **conversion of carbon dioxide gas to dry ice** (Option 3).

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